Process and apparatus for centering unground semiconductor rods

ABSTRACT

A process and apparatus for obtaining a cylinder of a specified diameter from an unground rod of varying diameters which includes focusing a beam of collimated light on the rod, masking the beam to control its width and to control the amount of light passing each side of the rod, and continuously manipulating and positioning the rod within the beam so as to move the centerline of the above cylinder into the same plane as that containing the centerline of the focussed beam.

[111 3,749,499 [451 July 31,1973

l,85l,l73 3/1932 Hall 356/164 PROCESS AND APPARATUS FOR CENTERINGUNGROUND SEMICONDUCTOR RODS Primary Examiner-Ronald L. Wibert AssistantExaminer-V. P. McGraw AuorneyHarold R. Patton [75] Inventors: Thomas E.Reichnrol, Kirkwood;

Thomas L. Bevel, Florissant, both of Mo.

[73] Assignee: Monsanto Company, St. Louis, Mo.

Filed: Dec. 27, 1971 ABSTRACT [2]] Appl' No: 212,265 A process andapparatus for obtaining a cylinder of a specified diameter from anunground rod of varying diameters which includes focusing a beam ofcollimated [52] US. 356/150, 356/l54, 356/159,

356/167 light on the rod, masking the beam to control its width Int.G01!) 1/00, GOlb 11/00 [58] Field of Search....................

[51 and to control the amount of light passing each side of the rod, andcontinuously manipulating and positioning 51/323 the rod within the beamso as to move the centerline of the above cylinder into the same planeas that containing the centerline of the focussed beam.

References Cited UNITED STATES PATENTS 3,470,384 Bodnas 356/168 12Claims, 6 Drawing Figures PATENIEDJULBI I975 3.749.499

sum 1 0F 3 FIG. I.

PATENIEL Jul 31 I975 SHEET 2 OF 3 FIG. 3.

a n w w I25 30 [I9 I05 m W ww w m FIG.4.

FIG. 2.

PATENIEIIJIIIBI I915 3'.74 9,499

SHEET 3 OF 3 I I FINAL INITIAL POSITION POSITION I CLI l I I To I 1 I58/I54 44 l I I 55 -46 I UL : ULZ ULII DMIN UL2 I I T i I l I I I I48 II44 g I I I --cLI I CLI I FIG. 5. FIG. 6.

PROCESS AND APPARATUS FOR CENTERING UNGROUND SEMICONDUCTOR RODS FIELD OFTHE INVENTION This invention relates generally to the preparation of 5BACKGROUND The Czochralski process for pulling semiconductor crystalsfrom a melt is well known in the art and has been used for many years togrow single crystals of silicon, germanium, and III-V intermetalliccompounds. In this process, a seed crystal is dipped into a moltencharge of semiconductive material and then slowly withdrawn at acontrolled rate to produce semiconductor rods having diameters typicallyranging from onehalf to 4 inches. Although a number of rod diametercontrol processes have been employed in the past in an attempt tomaintain substantially constant diameters in the pulled crystal rods,these monocrystalline rods nevertheless have diameter fluctuationsthroughout their lengths as a result of uncontrollable environmentalchanges in the crystal pulling process, such as changes in the meltlevel, temperature gradients in the melt and in the crystal pullingchamber and other environmental changes which cannot, as a practicalmatter, be maintained constant throughout the Czochralski process.Consequently, the crystal rods once pulled must be ground down to achosen uniform diameter using a grinding lathe.

In this grinding process, it is desirable to increase yields andminimize semiconductive material waste by reducing the crystal losseswhich accompany the process of grinding the rods down to specifieddiameters. Therefore, it is preferrable to be able to place the crystalrod in the grinding lathe in such a position as to grind the rod down toa cylinder of a desired specified diameter with a minimum of undersizelosses. When any length of the unground rod is undersize or has adiameter less than the specified diameter, then this much of the rodcannot be used for making wafers of the above specified diameter.However, the term undersize is used with reference to a specificlongitudinal centerline of the unground rod and with reference to thecylinder within the unground rod having this centerline and having thespecified diameter. For each unground semiconductor rod from which it isdesired to obtain the maximum length of a cylinder at a specifieddiameter, there are two cases. In the first case, there is no undersizein the unground rod, and within the entire length of the rod to beground, it is possible to obtain a perfect cylinder whose diameter issmaller than any single diameter ofthe unground rod. In this case, itonly becomes necessary to establish the centerline of this cylinder sothat the unground rod can be rotated in a grinder lathe about thiscenterline axis of rotation to grind the rod down to the above cylinderwith the specified diameter.

In the second case, the prescribed or predetermined diameter of thecylinder to be ground from the unground rod is larger than some of themany different diameters of the unground rod, and where the latter istrue then the unground rod is undersize with respect to the cylinder ofthe prescribed diameter. Therefore, in the latter case the selection ofthe centerline of the cylinder of prescribed diameter becomes veryimportant, because there will be a different amount of undersize lossfor each centerline selected. Thus, in this second case it is desirableto select the centerline of the unground rod for which undersize lossesare at a minimum.

in the past, skilled operators have tried to establish this centerlinewithin the unground rod either by visual inspection of the rod or byusing various types of centering templates for making center-pointmeasurements on each end of the rod. The visual inspection technique hasnot proved entirely satisfactory, since it is impossible with the humaneye to locate points on the ends of the unground rod which lie on theabove centerline axis. The centering template technique has been used tosatisfactorily locate the approximate center points of the area on eachend of the unground rod. However, the line which connects these twopoints is not necessarily the above identified desired centerline axisof rotation.

THE INVENTION In accordance with the present invention, a process andapparatus are provided wherein an optical masking system is employed tolocate the centerline axis of the cylinder within the ungroundsemiconductor rod having either no undersize losses or a minimum ofundersize losses. In practicing the present invention, the ungroundsemiconductor rod may be manipulated and positioned within a focussedcollimated light beam, and the width of this beam and the amounts oflight passing each side of the unground rod may be continuouslycontrolled by masking same during the manipulation and positioning ofthe rod. This beam has a centerline in the same plane as that of thedesired centerline sought to be established for the ungroundsemiconductor rod, so that by continuously manipulating and positioningthe rod on a support member during the reduction in light beam width,the undersize losses in the unground rod, if any, may be determined.Upon observation of these undersize losses during the manipulation andpositioning of the rod, the optimum centerline corresponding to aminimum of undersize losses can be established with a high degree ofaccuracy. Once this centerline is established, bearings may be removablysecured to the ends of the unground rod for rotation in a grindinglathe.

Accordingly, an object of the present invention is to provide a new anduseful process for rapidly and accurately centering semiconductor rodsin preparation for grinding same to a specified diameter with anabsolute minimum of semiconductive material waste.

Another object of this invention is to provide a new and useful rodcentering apparatus to facilitate the easy manipulation and handling ofan unground semiconductor rod while securing thereto rotational bearingson each end of the rod adaptable for rotation in a grinding lathe.

Another object of this invention is to provide a new and useful processand apparatus of the type described which is easy to construct andoperate, reliable in performance and relatively inexpensive in itsconstruction, upkeep and operation.

A further object of this invention is to provide a rod centlx ringprocess and apparatus of the type described which is operative toaccommodate desired centering of semiconductor rods having a variety ofshapes, such as flats, bows, dips, S-curves, hooks and the like.

These and other objects and features of the invention will becomeapparent in the following description of a preferred embodiment of theinvention.

DRAWINGS FIG. 1 is a prospective view, partially cut away, of the rodcentering apparatus embodying the invention;

FIG. 2 is a cross-section view taken along lines 2-2 of FIG. 1;

FIG. 3 is a greatly enlarged cross section view of the central portionof the turntable assembly in FIG. 2 and is shown with the rod centeringbearing thrust into adhesive contact with the semiconductor rod;

FIG. 4 is an enlarged view of the end portion of the semiconductor rodhaving the adhesive bearing secured thereto on the desired rodcenterline;

FIG. 5 is a mask and rod outline of the unground rod in its collimatedlight masking pattern for case one above where there are no undersizelosses, and

FIG. 6 is a mask and rod outline of the unground rod in its collimatedlight masking pattern for case two above where the unground rod is to becentered for maximum obtainable rod length and a minimum of undersizematerial losses.

DETAILED DESCRIPTION Referring now to FIG. 1, there is shown, inpartially broken away perspective view, rod centering apparatusdesignated generally as 10 and which includes a means to be describedfor focusing a beam 12 of collimated light on the semiconductor rod tobe centered. This beam 12 originates within a light box 14 having a slit16 therein through which a divergent light beam 17 passes, and a portionof the beam 17 passes through a Fresnel lens I8 mounted adjacent theopening 19 in the assembly housing 20. The collimated light beam 12 isinterrupted by the unground semiconductor rod 22 and split into a pairof parallel collimated beams 24 and 26 which are projected onto aluminescent screen 28. As will be described in detail below, thesemiconductor rod 22 is manipulated within the light beam 12 and onlower and upper rotary turntables 30 and 32, respectively, in order tocenter the semiconductor rod 22 in preparation for a subsequent grindingoperation. The manipulation and positioning of the ungroundsemiconductor rod 22 can be accomplished by means of adjusting screws34, and in accordance with the present invention the centerline CL ofthe semiconductor rod 22 is brought into the same plane with that of thecenterline CL of the masking members 44 and 46 for the collimated beam12 as well as the centerline CL, of the light beam l7. The vertical axisof rotation for the turntables 30 and 32 also lies in this verticalplane which is parallel to the collimated light beam 12.

The box housing 14 for the light source includes a light bulb 36 whichis mounted as shown on a support bracket 38, and the spring biasedscrews 40 may be adjusted to raise and lower and tilt the support plate42 and provide an adjustment for the light bulb 36 with respect to theslit 16. A cooling fan 39 is located beneath the bulb 36 to preventoverheating in the box housing 14.

The apparatus 10 also includes an adjustable light shield having thepair of masking members 44 and 46 vertically suspended as shown adjacentthe opening 19, and these masking members 44 and 46 may besimultaneously moved in a direction perpendicular to the collimatedlight passing therebetween in order to vary the width of this beam andthe amount oflight which passes each side of the semiconductor rod 22.The masking member 44 is fixedly mounted on the back of the coarsethread nut 70 and the masking member 46 is fixedly mounted on the hanger58, and the masking members 44 and 46 move toward and away from thecenter ofthe beam I2 with the movement ofnut 70 and hanger 58 to bedescribed.

The light shield assembly is securely mounted to the housing 20 by meansof a pair of support brackets 48, each having a bearing arm 50 and 52extending therefrom as shown for receiving an upper guide rail 54. Theupper guide rail 54 carries a pair of hanger members 56 and 58 which arefree to slide thereon, and a pair of lower guide rails 60 and 62 arefixedly mounted as shown between the hanger members 56 and 58 andbeneath the upper guide rail 54.

A mask adjusting thread screw 64 is threaded through an opening in thefixed support arm 66 and is received at one end in a shaft coupler 67.This screw 64 moves from right to left as viewed from the front ofassembly 10 and through screen 28 to thereby move the hanger 58 fromright to left, sliding on upper guide rail 54. The shaft coupler 67 alsoreceives the thread screw which extends through the opening 68 in thehanger 58 and is threaded through the center of the coarse thread nut 70which carries the masking member 44. The thread screw 65 is keyed to thehanger 58 so that it can rotate but not move through the hanger 58 fromleft to right as viewed from the front of assembly It). The coarsethread nut 70 has a pair of openings 72 and 74 therein through which thelower guide rails 60 and 62 pass, so that the thread nut 70 is moved onthread screw 65 horizontally from left to right on the guide rails 60and 62 by turning the adjusting screw 64. This movement simultaneouslymoves hanger 58 and masking member 46 from right to left, and thismovement will be at the same rate as that of hanger 58 if the threadpitch of screw 65 is twice that of screw 64. Therefore, by turning theknob 76, the nut 70 and hanger 68 may be simultaneously moved toward oraway from the center of the light beam 12 at the same rate.

The lower turntable 30 is mounted on the bottom wall 78 of the apparatus10, and the upper turntable 32 is mounted on the support bracket 80which is held in position by a pair of vertical guide rails 82 and 84.The bracket 80 may be raised or lowered from the position shown in FIG.1 by means of a pulley assembly which includes a pair of weights 86suspended from a pair of ropes 88 and 90. These ropes are movable aroundthe pulley arms 92 and 94 which are mounted as shown on the back wall ofthe assembly 10. The turntable assembly portion of the apparatus 10 canbe seen more clearly by referring to the cross section views of FIGS. 2and 3 which illustrate the rotatable mounting assemblies of both thelower and upper turntables 30 and 32.

Referring now to FIG. 2, the lower turntable 30 and associated mountingassembly therewith will be described. The upper and lower turntableassemblies are substantially identical, and therefore only the lowerturntable 30 and its related mounting assembly will be described hereinwith reference to FIG. 2. The lower turntable 30 is mounted for rotationabout a preselected centerline axis by means of ball-bearings whichinclude an inner race member 95 secured to the turntable 30 and an outerrace member 96 secured to a bearing housing 98. The turntable 30 may befreely rotated 360 by means of these ballbearings, and the outer bearinghousing 98 is in turn securely clamped to the bottom wall 78 of theassembly by means of a clamp ring 107. This outer clamp ring 107 has itsinner peripheral edge in contact with the bottom wall of the outer racemember 96 and thereby firmly clamps the outer race member 96 to thebottom wall 78 of the assembly 10. An inner clamp ring or spacer 106 ispositioned in contact with the bottom wall of the inner race member 95,and by tightening a lock nut 102 against the spacer 106, the inner racemember 95 is held securely against the turntable 30. Note that the locknut 102 does not contact the outer clamp ring 107, there being an airgap between the latter two members. The spacer 106, therefore, keeps thelock nut 102 separated from the outer clamp ring 107.

The unground semiconductor rod 22 is initially brought to rest on arubber pad 105 on the upper surface 108 of the rotary turntable 30. Theturntable 30 carries on its outer edge, and at 90 spacings, fouradjustable screw brackets 110 which threadedly engage, respectively, thefour previously identified adjusting screws 34. Thus, in centering therod 22, an operator may freely rotate the turntable 30 to any one offour different 90 positions and make from one to four adjustments of thescrews 34 at each of these 90 positions in order to move thesemiconductor rod 22 in the X and Y directions as desired on the rubberpad 105. The adjusting screws 34 each carry swivel pads 112 on the tipsthereof, and these pads are brought into resilient contact as shown withthe ends of the semiconductor rod 22 being centered.

An air cylinder 114 is secured to the lower end of the turntable 30 bymeans of a screw 115, and another smaller screw 117 is slidablypositioned centrally of the screw 115 and is free to move verticallythrough the screw 115 so as to move the entire chuck 118 upwardly towardthe bottom of the semi-conductor rod 22. The smaller screw 117 may beconnected at its lower end to any member (not shown) suitable forefficiently transmitting air pressure to it from the air cylinder 114.

The chuck 118 has a recess in the upper portion thereof for receiving aresilient pad 120 which has an opening in its center for receiving aso-called hex-ball member 119. The rod centering bearing 125 is mountedon this hex-ball member I19 which is in the general shape of a sphere,but has hexagonal outer sections extending from its top as shown in FIG.2 down to a stem 121 which extends into the narrow central cylindricalrecess of the chuck 118. The bearing 125 has its inner recess contouredto the'shape of the hex-ball member 119, so that the bearing 125 may berotary driven by a similar hex-ball type member (not shown) once thebearing 125 is secured to the end of the rod 22.

Referring now to FIGS. 2, 3 and 4 a pair of rollers, such as spheres 132and 134, are held in place as shown adjacent the outer edge of thebearing 125 by means of an O-ring seal 136, and these rollers facilitatethe firm and smooth transmittal of vertical motion from the chuck 118 tothe bearing 125.

After the rod 22 has been centered in accordance with the presentinvention, air pressure is introduced by any suitable means into the aircylinder 114 to force the inner screw 117 upwardly through itssurrounding outer screw member 115 to thereby move the chuck 118vertically toward the bottom of the rod 22. Once the adhesive 128 on theupper surface of the bearing 125 comes into contact with the rodsurface, then the bearing 125 is free to move against the lowerresilient pad 120. Similarly, the upper resilient pad 122 gives with themovement of the upper bearing 126. This movement permits the adhesive128 to assume the contour of the bottom end of the semiconductor rod 22,and during this upward movement of the bearing 125, this bearing isguided upwardly by the rollers 132 and 134. If for some reason the rod22 has been sawed at an angle such that its surface 130 is not in aplane parallel to the plane of the surface of the adhesive 128, thenthis does not prevent accomplishing a good secure adhesive bond betweenthe rod 22 and adhesive 128 when the bearing 125 is thrust upwardly intocontact with the lower surface 130 of the rod 22.

The structure in FIG. 3 differs from that shown in FIG. 2 in that aspherical centering ball 124 rather than a hex-ball is used to transmitthe vertical force to the bearing 126, and the centering ball 124 has astem 127 thereon which extends into the narrow cylindrical vertical slotof the chuck 118. Thus, as shown in FIG. 4, the bearing 126 which issecured to the surface 130 of the semiconductor rod 22 has ahemispherical recess 129 therein which is free to rotate on a sphericalor hemispherical member of the same shape. Thus, the rod 22 is providedwith a bearing 126 on one end thereof which is freely rotatable in oneend of a grinding machine and a bearing 125 secured to the other endthereof which has a hexagonally shaped inner recess. Such recess enablesthe bearing 125 to be driven by a drive shaft in the other end of thegrinding machine and having either an end or a member attached to itsend with a contoured hexagonal shape.

Referring now to FIG. 5, the present invention is illustrated for theabove described case one wherein there is no single diameter of theunground rod 22 smaller than the target diameter TD to which the rod 22will be ground. For this case, an operator will place the rod 22 on theturntables 30 and 32 as described above and continuously rotate theseturntables while making the appropriate adjustments of the eight screws34, four at the bottom and four at the top, at each position. A vernieror scale (not shown) is positioned adjacent the edges of the blades 44and 46 and is set at the target diameter TD, so that the spacing betweenthese blades can be observed with respect to the target diameter. Therod 22 is moved in the X and Y directions and simultaneously rotatedwhile the edges of the mask 44 and 46 are moved inwardly until lightpassing the rod 22 is just closed off at locations 140, 142 and 144along the edges of the rod 22. These points are thus lined up with theedges of the mask 44 and 46. Thus, for case one above, there is a rodcentering alignment dimension T which will be removed during thesubsequent grinding process in order to grind the rod 22 down to thetarget diameter, TD. The centerline CL, of the cylinder having thetarget diameter TD is, of course, the centerline for a cylinder with amaximum diameter, D which is the maximum diameter rod that could beobtained from the unground rod 22. Therefore, if the object in thesubsequent grinding process is to obtain a cylinder of maximum diameter,D from the unground rod 22, then the grinding machine will be set togrind the rod 22 at this diameter.

Referring now to FIG. 6 there is illustrated the above described casetwo where the unground rod 22 has undersized regions therein withdiameters less than the target diameter TD. In this case, the spacing ofthe blades 44 and 46 is reduced to the target dimension TD and nofurther; and for this spacing, light will pass by the edges of thesemiconductor rod 22 and be received on the luminescent screen 28 (FIG.1). In initially positioning the rod 22, assume that it is moved to afirst position with respect to the masks 44 and 46 so that the edges ofthe rod 22 visually intersect the right hand mask 46 edge at points 146and 148 and intersect the left hand mask 44 edge at points 150 and 152.For this position, there is an undersize loss UL observed on the righthand edge of the rod 22 and an undersize loss UL, on the left hand sideof the rod 22 which is within the undersize loss UL,. Therefore, if therod 22 is centered at the above position, the length of rod UL would beundersized with respect to the target diameter TD and therefore would belost in a subsequent grinding operation using this initial centerline.

In order to reduce this undersize loss UL the operator will, whilesimultaneously rotating the rod 22 to each of the 90 positions (orintermediate positions)of screws 34 on the turntables 30 and 32, movethe rod 22 (as viewed in FIG. 6) to the right with respect to the fixedmasks 44 and 46 until the rod 22 now intersects the mask at points 154and 156 on its right hand edge corresponding to a new, smaller undersizeloss UL For ease of illustration in FIG. 6 of the drawings, there areshown two sets of mask edges and a single rod. But this should not beconfused with any movement of the mask edges 44 and 46, since theseedges remain fixed during the rod centering process while the rod 22 ismoved to the right. However, in order to best illustrate this relativemotion and a reduction in undersize losses, the above movement of therod 22 is represented by the new points 154 and 156 of intersection ofthe right hand edge of the rod 22 with right hand mask 46 and new pointsof intersection 158 and 160 with the left hand mask 44. This newposition of rod 22 corresponds to the new undersize losses UL betweenpoints 1154 and 156 and UL between points I58 and 160, so that the newundersize loss UL on the left hand side of the rod 22 is nowapproximately equal to UL At this point in the rod centering process,the operator can tell that any further shifting of the rod 22 to theright, while decreasing UL will now begin to increase UL Thus, a pointof minimum undersize loss has been reached and corresponds to thecenterline CL,; and index marks or bearings, such as the bearings 125and 126 illustrated in FIG. 2 and 3 may be applied to the ends of therod 22 in preparation for subsequent rotation and grinding about thecenterline.

If, of course, the alignment objective should be to obtain a completecylinder having its length equal to that of the unground rod and amaximum diameter for such length, then this diameter would be theminimum diameter, D,,,,,,, as shown in FIG. 6. For this centering anoperator will further reduce the masked width of the light beam andposition the rod 22 until the mask edges 44 and 46 just meet the edgesof the rod 22 at points [62 and 1164 thereon at which all light passingby the rod edges is masked or closed off.

Various modifications may be made in the above described embodiments ofour invention without departing from the true scope thereof. Forexample, the masks 44 and 46 could be located between the rod and theoperator instead of between the light source and the rod; and inaddition, these masks could be fixed in either of the above positionsrather than moveable if only one specified target diameter is ofinterest. Furthermore, these masks, either fixed or movable could bemade of a transparent material having either a straight line indicatingthe target diameter or a contoured line corresponding to a desiredrotational shape other than a straight cylinder. Or, in the alternative,the mask could be partly transparent, so that the complete rod outlineshows on a screen, both over and under a target line. Additionally, themask could be colored with various colors corresponding to undersize andoversize portions of the rod.

It should be further pointed out that the present invention is notlimited to a rod centering process for use only with grinders, but, ifdesired, the present rod centering process can be used in preperation ofrods for various types of lapping or sanding machine tools which rotatea centered object about a particular rotational axis of the machinetool. Furthermore, the invention is not limited to the formation of rodshaving cylindrical shapes, and may be utilized in preparing rods for agrinding operation wherein the said rods are cut to various othershapes. It will also be understood and appreciated by those skilled inthe art that the present invention may be fully automated by, forexample, the use of photo detectors in a closed-loop servo systemwherein the photo detectors sense the light passing on each side of therod and in response thereto generate a differential signal. Such signalmay be processed in a closed-loop servo system to provide the mechanicalX-Y positioning movement necessary to position the rod on theturntables. Accordingly, it will be understood that the presentinvention is not limited to the above described preferred embodimentthereof.

We claim:

1. A process for locating a centerline for a selected geometrical shapewithin an irregularly shaped object to be ground about said centerlineto a chosen geometric shape in a grinding machine, which processincludes:

a. locating said object so as partially to interrupt a beam of light ofpredetermined width provided by a light source, said beam having acenterline,

b. selectively varying said predetermined width by variably masking saidbeam between an observation area and the source of said light beam, and

c. shifting said object with respect to said light beam while rotatingsaid object within said beam so as substantially to equalize the amountsof light passing by opposite sides of said object and reaching saidobservation area, whereby after said equalizing the centerline of saidobject is aligned with the centerline of said light beam and therebyidentified, whereby said object may be rotated about the so-identifiedcenterline in a grinding machine or the like to grind said object to achosen geometrical shape.

2. The process defined in claim 1 wherein said shifting is carried outwhile simultaneously varying the width of said beam, so that thecenterline established for s;:.id object is also the centerline of acylinder within said irregularly shaped object.

3. A process for obtaining the centerline ofa cylinder ofa specifieddiameter from an unground semiconductor rod consisting of many differentdiameters, which process includes:

a. locating the semiconductor rod so as partially to interrupt a beam oflight of predetermined width provided by a light source;

b. varying said predetermined width by masking said beam between thesource of said light beam and said rod, and

c. shifting said rod with respect to said light beam while rotating saidrod within said beam so as to equalize the amounts of light passing byopposite sides of said rod, whereby an optimum centerline in saidunground rod for minimum undersize grinding losses is therebyestablished.

4. The process defined in claim 3 wherein said shifting said rod iseffected by lateral positioning of its opposite ends while varying thewidth of said beam.

5. A process for determining the central axis of a cylinder of aspecified diameter within an unground semiconductor rod of varyingdiameters comprising:

a. providing by means of a light source a collimated light beam ofpredetermined initial width,

b. locating said unground rod in the path of said collimated light beamand near the centerline thereof, said rod having a diameter less thanthe initial width of said beam and partially interrupting said beam anddividing it into two separate beams of collimated light,

c. masking said beam of said collimated light between said rod and thesource of collimated light to thereby vary the amounts of light passingeach side of said unground rod,

d. continuously repositioning said rod while continuously diminishingthe width of said light beam to maintain the widths of said two beamsapproximately equal at given points along the length of said rod,

e. thereafter diminishing the width of said two beams to or toward zeroat given locations along the length of said rod, whereby the centerlineof said cylinder may be brought into the same plane as that containingthe centerline of said light beam, and

f. affixing index means such as rotational bearings on each end of saidunground rod and on said centerline of said cylinder, whereby said rodmay then be ground down to a cylinder having said specified diameter andsaid undersize losses reduced or completely eliminated.

6. The process defined in claim 5 wherein the continuous repositioningof said rod includes rotating said rod while varying the width of saidbeam.

7. Rod centering apparatus for establishing a centerline in an ungroundrod for a cylinder therein of a prescribed diameter, including incombination:

a. means for focusing a beam of collimated light towards said ungroundrod so that said rod partially interrupts said beam of light and allowsseparate beams of collimated light to pass said rod on each sidethereof,

b. means for supporting said rod and having a centerline in the sameplane as the centerline ofsaid collimated light beam,

0. means for diminishing toward zero the light passing each side of saidrod at one or more points along the length thereof to enable thecenterline of said cylinder to be moved into the same plane with thecenterline of said collimated light beam, and

d. rod positioning means for moving said rod in the X and Y directionson said supporting means, so that the amounts of light passing on eachside of said rod may be varied.

8. Apparatus defined in claim 7 which further includes light imagingmeans mounted adacent said supporting means and forming images of thetwo separate collimated light beams passing by each side of said rod,whereby said diminishing small amounts of light passing by each side ofsaid rod may be visually observed and maintained substantially equal assaid amounts are reduced to or toward zero as said unground rod ismanipulated and positioned on said supporting means.

9. Apparatus defined in claim 7 wherein said supporting means includes apair of tables rotatably mounted to receive said unground rod on theopposite ends thereof and each having a centerline axis of rotation inthe same plane as the centerline axis of said collimated light beam,whereby when the small amounts of light passing said rod are diminishedto zero, said cylinder has its centerline moved into the same plane andcoincident with the rotational axis of said tables and can be markedthereon in preparation for a subsequent grinding operation. 1

10. Apparatus defined in claim 7 wherein said diminishing means includesmasking means located between said focusing means and said supportingmeans, said masking means being symmetrically mounted with respect tosaid plane and including means for increasing or decreasing the amountsof light passing each side of said rod.

11. Apparatus defined in claim 9 wherein said tables each includes meanstherein for thrusting an adhesive carrying bearing toward the ends ofsaid rod and on said centerline of said cylinder thereby to facilitatethe mounting of said rod in a lathe for grinding said rod down to saidcylinder of said prescribed diameter.

12. Apparatus defined in claim 8 wherein:

a. said supporting means includes a pair of tables rotatably mounted toreceive said unground rod on the opposite ends of the rod and each tablehaving a centerline axis of rotation in the same plane as the centerlineaxis of said collimated light beam, whereby when the small amounts oflight passing said rod are diminished to zero, said cylinder has adesired centerline thereof moved into the same plane and coincident withthe rotational axis of said tables and can be marked thereon inpreparation for a subsequent grinding operation,

b. said diminishing means includes masking means located between saidfocusing means and supporting means, said masking means beingsymmetrically mounted with respect to said plane and including means forincreasing or decreasing the amounts of light passing each side of saidrod, and

c. said tables each includes means therein for thrusting an adhesivecarrying bearing toward the ends of said rod and on the said centerlineestablished therein to facilitate the easy mounting of said rod in agrinding machine or the like for grinding said rod down to a prescribeddiameter or other chosen geometrical shape symmetrical about saidcenterline.

1. A process for locating a centerline for a selected geometrical shapewithin an irregularly shaped object to be ground about said centerlineto a chosen geometric shape in a grinding machine, which processincludes: a. locating said object so as partially to interrupt a beam oflight of predetermined width provided by a light source, said beamhaving a centerline, b. selectively varying said predetermined width byvariably masking said beam between an observation area and the source ofsAid light beam, and c. shifting said object with respect to said lightbeam while rotating said object within said beam so as substantially toequalize the amounts of light passing by opposite sides of said objectand reaching said observation area, whereby after said equalizing thecenterline of said object is aligned with the centerline of said lightbeam and thereby identified, whereby said object may be rotated aboutthe so-identified centerline in a grinding machine or the like to grindsaid object to a chosen geometrical shape.
 2. The process defined inclaim 1 wherein said shifting is carried out while simultaneouslyvarying the width of said beam, so that the centerline established forsaid object is also the centerline of a cylinder within said irregularlyshaped object.
 3. A process for obtaining the centerline of a cylinderof a specified diameter from an unground semiconductor rod consisting ofmany different diameters, which process includes: a. locating thesemiconductor rod so as partially to interrupt a beam of light ofpredetermined width provided by a light source; b. varying saidpredetermined width by masking said beam between the source of saidlight beam and said rod, and c. shifting said rod with respect to saidlight beam while rotating said rod within said beam so as to equalizethe amounts of light passing by opposite sides of said rod, whereby anoptimum centerline in said unground rod for minimum undersize grindinglosses is thereby established.
 4. The process defined in claim 3 whereinsaid shifting said rod is effected by lateral positioning of itsopposite ends while varying the width of said beam.
 5. A process fordetermining the central axis of a cylinder of a specified diameterwithin an unground semiconductor rod of varying diameters comprising: a.providing by means of a light source a collimated light beam ofpredetermined initial width, b. locating said unground rod in the pathof said collimated light beam and near the centerline thereof, said rodhaving a diameter less than the initial width of said beam and partiallyinterrupting said beam and dividing it into two separate beams ofcollimated light, c. masking said beam of said collimated light betweensaid rod and the source of collimated light to thereby vary the amountsof light passing each side of said unground rod, d. continuouslyrepositioning said rod while continuously diminishing the width of saidlight beam to maintain the widths of said two beams approximately equalat given points along the length of said rod, e. thereafter diminishingthe width of said two beams to or toward zero at given locations alongthe length of said rod, whereby the centerline of said cylinder may bebrought into the same plane as that containing the centerline of saidlight beam, and f. affixing index means such as rotational bearings oneach end of said unground rod and on said centerline of said cylinder,whereby said rod may then be ground down to a cylinder having saidspecified diameter and said undersize losses reduced or completelyeliminated.
 6. The process defined in claim 5 wherein the continuousrepositioning of said rod includes rotating said rod while varying thewidth of said beam.
 7. Rod centering apparatus for establishing acenterline in an unground rod for a cylinder therein of a prescribeddiameter, including in combination: a. means for focusing a beam ofcollimated light towards said unground rod so that said rod partiallyinterrupts said beam of light and allows separate beams of collimatedlight to pass said rod on each side thereof, b. means for supportingsaid rod and having a centerline in the same plane as the centerline ofsaid collimated light beam, c. means for diminishing toward zero thelight passing each side of said rod at one or more points along thelength thereof to enable the centerline of said cylinder to be movedinto the same plane with the centerline of said collimated light beam,and d. rod positioning means for moving said rod in the X and Ydirections on said supporting means, so that the amounts of lightpassing on each side of said rod may be varied.
 8. Apparatus defined inclaim 7 which further includes light imaging means mounted adacent saidsupporting means and forming images of the two separate collimated lightbeams passing by each side of said rod, whereby said diminishing smallamounts of light passing by each side of said rod may be visuallyobserved and maintained substantially equal as said amounts are reducedto or toward zero as said unground rod is manipulated and positioned onsaid supporting means.
 9. Apparatus defined in claim 7 wherein saidsupporting means includes a pair of tables rotatably mounted to receivesaid unground rod on the opposite ends thereof and each having acenterline axis of rotation in the same plane as the centerline axis ofsaid collimated light beam, whereby when the small amounts of lightpassing said rod are diminished to zero, said cylinder has itscenterline moved into the same plane and coincident with the rotationalaxis of said tables and can be marked thereon in preparation for asubsequent grinding operation.
 10. Apparatus defined in claim 7 whereinsaid diminishing means includes masking means located between saidfocusing means and said supporting means, said masking means beingsymmetrically mounted with respect to said plane and including means forincreasing or decreasing the amounts of light passing each side of saidrod.
 11. Apparatus defined in claim 9 wherein said tables each includesmeans therein for thrusting an adhesive carrying bearing toward the endsof said rod and on said centerline of said cylinder thereby tofacilitate the mounting of said rod in a lathe for grinding said roddown to said cylinder of said prescribed diameter.
 12. Apparatus definedin claim 8 wherein: a. said supporting means includes a pair of tablesrotatably mounted to receive said unground rod on the opposite ends ofthe rod and each table having a centerline axis of rotation in the sameplane as the centerline axis of said collimated light beam, whereby whenthe small amounts of light passing said rod are diminished to zero, saidcylinder has a desired centerline thereof moved into the same plane andcoincident with the rotational axis of said tables and can be markedthereon in preparation for a subsequent grinding operation, b. saiddiminishing means includes masking means located between said focusingmeans and supporting means, said masking means being symmetricallymounted with respect to said plane and including means for increasing ordecreasing the amounts of light passing each side of said rod, and c.said tables each includes means therein for thrusting an adhesivecarrying bearing toward the ends of said rod and on the said centerlineestablished therein to facilitate the easy mounting of said rod in agrinding machine or the like for grinding said rod down to a prescribeddiameter or other chosen geometrical shape symmetrical about saidcenterline.